This invention relates to a method of producing glass products such as an optical glass and, in particular, to a method of producing glass products in which final glass products are obtained through a press-molding step using a mold and a polishing step subsequent thereto. This invention also relates to an apparatus for producing the glass products.
Generally, an optical glass as one of glass products can be produced by the following three methods.
A molten glass is press-molded by the use of a molding die to directly form a lens blank. Thereafter, the lens blank is polished.
A molten glass is poured into a molding die to be molded into a prism-shaped or a plate-shaped glass block. The glass block is cut by mechanical machining into small cut pieces. The cut pieces are reheated and pressed to produce pressed products. Finally, the pressed products are subjected to polishing.
A molten glass is formed into a preform having a shape approximate to that of a final product. The shape and surface precision of a mold is transferred to the preform to produce the final product.
Among the above-mentioned methods, the third method is free from polishing and is therefore adapted to production of a lens, such as an aspheric lens, having a shape difficult to polish. However, restriction is imposed upon a glass material which can be used in this method. Specifically, the viscosity of the glass material upon pressing must fall within a predetermined range between 108 and 1012 poises. Furthermore, a large-scale and expensive production facility is required. This results in an increase in production cost.
Therefore, in order to produce a large amount of glass products at a reduced production cost, the first and the second methods are preferred.
Now, comparison will be made between the first and the second methods. The first method is adapted to small-item large-volume production but is not adapted to large-item small-volume production. This is because the molten glass must immediately be shaped into the glass blank. On the contrary, the second method is adapted to large-item small-volume production but is not adapted to small-item large-volume production. According to the second method, plate-shaped glass blocks of a plurality of kinds of glass materials can be preliminarily prepared and stored. When desired, a selected one of the plate-shaped glass blocks can be cut into cut pieces to be pressed.
Typically, the above-mentioned optical lens has been used in an optical apparatus, such as a camera using a photographic paper. In addition, the optical lens recently becomes widely used in electronic products (for example, a digital camera, a video camera, and a recording/reproducing device for a recording medium). With the arrival of a multi-media society, the volume of production of the optical lens used in the electronic products exhibits a drastic increase.
In order to enhance competitive power of a manufacturer of the optical glass, it is required to shorten a turn-around time (TAT) of the products in large-item large-volume production and to reduce the production cost. For this purpose, the second method has a potential success among the conventional methods mentioned above. For example, the second method is described in Japanese Unexamined Utility Model Publication (JP-U) No. H02-142439.
However, in the second method, the cut pieces to be reheated and pressed are obtained by molding the molten glass into the glass block of a plate-like shape and cutting the glass block. Therefore, it is difficult to suppress the variation in weight among the cut pieces.
This is because the weight control is performed at the time when the molten glass is poured into the molding die and molded into the plate-like shape. When the plate is cut into a plurality of blocks, the variation in weight is inevitably produced among the individual blocks. Furthermore, when the molten glass is molded into the plate-like shape, it is difficult to form a plate having a uniform thickness. This also contributes to the variation in weight.
Due to such variation in weight, it is impossible to reduce the grinding amount of the lens in the final polishing step. Therefore, the amount of a scrap material can not be decreased so that an adverse influence upon the environment can not be suppressed. At present, the technical progress in electric products is very rapid and even a new product becomes very quickly old-fashioned and disposed of. Under the circumstances, the influence of the scrap material upon the environment is a serious problem which can not be neglected.
Furthermore, since a glass mass is obtained by mechanical machining, limitation is imposed upon the reduction in production cost.
The use of the prism-shaped material results in another disadvantage. Specifically, during reheating thermal nonuniformity is caused between the corners and the center of the prism-shaped material. Therefore, upon pressing by the use of the mold, reproducibility with respect to the mold is deteriorated.
It is therefore an object of this invention to provide a method and an apparatus for producing glass products, which are capable of suppressing the production cost and which are adapted to large-item large-volume production.
It is another object of this invention to provide a method and an apparatus for producing glass products, which can suppress an adverse influence upon the environment.
It is still another object of this invention to suppress thermal nonuniformity when cut pieces are reheated and press-molded.
According to this invention, there is provided a method of producing glass products, comprising a glass mass forming step of forming glass masses by receiving a molten glass in a plurality of molding dies and gradually cooling the molten glass, a pressing step of reheating the glass masses in an air atmosphere to a temperature corresponding to a viscosity of 104-106 poises and press-molding the glass masses by a mold to produce press-molded products, and a polishing step of polishing the press-molded products to form final glass products.
Preferably, the molten glass in the glass mass forming step has a viscosity of 30-2 poises.
Preferably, the glass mass is made of a glass having a glass transition point Tg of 580xc2x0 C. or more.
Preferably, the pressing step is performed by reheating the glass mass after subjected to polishing to increase the surface roughness.
Preferably, the pressing step is performed by applying a powdery parting agent on the surface of the glass mass after subjected to polishing.
Preferably, the pressing step is performed by reheating the glass mass by heat radiation.
Preferably, in the glass mass producing step, the molten glass continuously supplied is received by the molding dies heated to a predetermined temperature and successively fed, and is molded to form the glass masses.
Preferably, the molten glass is cut when the weight of its lower end portion flowing down exceeds the surface tension of the molten glass.
Preferably, the molten glass is molded in the molding dies into the glass masses in a floating or a substantially floating state.
Preferably, a cutting time of the molten glass is not longer than 1.0 second.
The glass products may be optical products.
According to this invention, there is also provided a method of producing press-molded products, the method being for distributing a plurality of glass masses to a plurality of molds and press-molding the glass masses to obtain press-molded products of different shapes, the method comprising the steps of a glass mass forming step of receiving a predetermined weight of molten glass in a molding die to form a glass mass, repeating the glass mass forming step to produce a plurality of glass masses, and distributing the glass masses to the molds, and press-molding by each mold the glass masses reheated to a temperature corresponding to a viscosity between 104 and 106 poises to obtain the press-molded products of different shapes.
Preferably, the molding die receives the molten glass in a floating or a substantially floating state to form the glass mass.
According to this invention, there is also provided a method of producing glass products, comprising the steps of producing press-molded products by the use of the above-mentioned method and polishing the press-molded products to obtain final glass products. The glass products may be optical products.
According to this invention, there is also provided an apparatus for producing a glass mass, the apparatus realizing a glass mass forming step in the above-mentioned method of producing glass products, the apparatus comprising a molten glass supply unit for supplying a molten glass through a nozzle, a molding unit for molding the molten glass, and heating means for heating the molding die and the molten glass to a predetermined temperature, wherein the molding unit includes a plurality of molding dies having molding surfaces for molding the molten glass supplied thereto into a predetermined shape, and transferring means for successively transferring the molding dies one by one to a lower end portion of the nozzle in an indexed manner.
According to this invention, there is also provided an apparatus for producing a glass mass, the apparatus comprising a molten glass supply unit for supplying a molten glass through a nozzle, a molding unit for molding the molten glass, and heating means for heating the molding dies and the molten glass to a predetermined temperature, the molding unit including a plurality of molding dies having molding surfaces for molding the molten glass supplied thereto into a predetermined shape, and transferring means for successively transferring the molding dies to a lower end portion of the nozzle in a continuous manner.
Preferably, the transferring means is a turntable rotated by driving means.
Preferably, the driving means for driving the turntable has a rotation speed controlled with reference to the viscosity of the molten glass.
Preferably, the molding surface of the molding die is provided with at least one gas supply hole for floating up the molten glass.
Preferably, each of the molding dies is arranged to be movable away from and towards the nozzle, the molding die being controlled to move towards the nozzle upon receiving the molten glass and to move away from the nozzle upon cutting the molten glass being received.
According to this invention, there is also provided a method of producing glass products, comprising the steps of forming glass masses by receiving a molten glass in a plurality of molding dies and gradually cooling the molten glass and reheating the glass masses in an air atmosphere to a temperature corresponding to a viscosity of 104-106 poises and press-molding the glass masses by the mold to produce press-molded products.
Preferably, the above-mentioned method further comprises the step of polishing the press-molded products to form final glass products.